Continuous casting of metal sheets and bands

ABSTRACT

The invention provides an apparatus for the continuous casting of metal sheets and bands, including two rotatable roll-crystallizers having parallel axes located in the horizontal plane and defining between them a gap of adjustable width, and means for supplying metal melt to fill the gap; characterized by means for generating at least three substantially horizontal magnetic fields producing electromagnetic pressures adapted to act to contain at least one end face of the melt within the gap.

FIELD OF THE INVENTION

[0001] The present invention relates to the casting of metal sheets and bands, and more particularly, to the continuous vertical casting of sheets between rotating roll-crystallizers.

BACKGROUND OF THE INVENTION

[0002] The production of thin sheets from various metals and alloys, using traditional methods, is one of the most power-consuming technological processes in ferrous and non-ferrous metallurgy. Striving to reduce the high costs of such sheets, metallurgists reverted to the idea proposed by Bessemer as early as 1847. However, the application of this idea to high-temperature melts has become possible only during the last 30 years, after it was proposed to use electromagnetic methods for a sidewall containment of the melt in the working gap between rolls.

[0003] There exist three U.S. Patents, largely based on the theoretical and experimental work of Kapusta, et al., carried out in the U.S.S.R. in the early 1970's: U.S. Pat. No. 4,936,374 (1990), which proposes the application of a horizontal, high-frequency magnetic field for metal containment in the working gap between rolls; U.S. Pat. No. 4,974,661 (1990), which suggests a vertical, high-frequency magnetic field, and U.S. Pat. No. 5,495,886 (1996), featuring three vertical magnetic fields— a DC field, an AC high-frequency field, and an AC low-frequency field.

[0004] Analysis of the prior art, based on extensive experience in electromagnetic metal containment, leads to the following conclusions:

[0005] The use of one horizontal or vertical, high-frequency magnetic field affords control of only one parameter and does not ensure the stable containment of metal, e.g., steel, at heavier liquid layer thicknesses for relatively thick layers.

[0006] While, under certain conditions, the use of three vertical magnetic fields and a horizontal current density field is possibly able to ensure stable metal containment, this is conditional upon the simultaneous control of four electromagnetic parameters, which casts serious doubt on the practicability of such control. Moreover, the design of an apparatus for carrying out such a method is extremely complex.

[0007] Also, all of the prior art patent refer to the apparatus used by inference only.

DISCLOSURE OF THE INVENTION

[0008] It is thus one of the objects of the present invention to provide an apparatus for the continuous, vertical casting of metal sheets and bands that is mechanically relatively simple and that comprises an arrangement of coils for generating strategically located magnetic fields which ensure the sidewall containment of the gap between rolls that defines the thickness of the cast sheet or band and enhances the stability and smoothness of the surface of the sheet or band being cast.

[0009] According to the invention, the above object is achieved by providing an apparatus for the continuous casting of metal sheets and bands, comprising two rotatable roll-crystallizers having parallel axes located in the horizontal plane and defining between them a gap of adjustable width, and means for supplying metal melt to fill said gap; characterized by means for generating at least three substantially horizontal magnetic fields producing electromagnetic pressures adapted to act to contain at least one end face of said melt within said gap.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

[0011] With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

[0012] In the drawings:

[0013]FIG. 1 is an elevational view of the apparatus according to the present invention, showing, on the left of the vertical center line, a cross-sectional view of one of the rolls, and, on the right, a view in the direction of arrows AA in FIG. 2;

[0014]FIG. 2 is a side view of the apparatus as-seen in the direction of arrow B in FIG. 1;

[0015]FIGS. 3a-c represent three variants of the jacket enveloping the core;

[0016]FIG. 4 represents, to a larger scale, detail D of FIG. 5;

[0017]FIG. 5 is a perspective view of the C-shaped, high-frequency electromagnet;

[0018]FIG. 6 is a schematic top view of the apparatus, illustrating the arrangements of the magnetic fields and the connections of the various power supplies;

[0019]FIG. 7 is a perspective view of one of the two supporting beams of the roll assembly, and

[0020]FIG. 8 is a perspective view of the laminate arrangement of the expanding magnetic cores.

DETAILED DESCRIPTION

[0021] Referring now to the drawings, there is shown in FIG. 1, on the left of the vertical center line, a cross-sectional view of one of two roll-crystallizers 2, (hereinafter referred to as roll or rolls 2), having parallel axes lying in a horizontal plane. Each of the rolls 2 comprises a core 4 made of non-magnetic steel with ducts 6 for the circulation of a liquid coolant; two stub axles 8 made of magnetic carbon steel with a central bore 10 communicating with ducts 6; jacket 12, made of a thermally highly conductive material, e.g., copper, and adapted to be cooled by the liquid coolant.

[0022] Jacket 12 is shown to better effect in FIGS. 3a-c, which represent three possible variants of the jacket. A basic jacket 12 is shown in FIG. 3a, the inside surface of jacket 12 being provided with a plurality of equally spaced, axially directed grooves 13 which end in a manifold on both sides of roll 2 (FIG. 4). A similar cooling effect can also be achieved by helical grooves 13. FIGS. 3b and 3 c show variants in which the outer surface of jacket 12 is provided with U-profiled and trapezoidal grooves, respectively, into which are inserted electrically insulated copper bars 14, 14 a, having appropriate cross-sections. The purpose of these bars will become apparent further below.

[0023] The connection between roll cores 4, stub axles 8 and jacket 12 is shown to better effect in FIG. 4, where it is seen that cores 4 are connected to flanges 15 of stub axles 8 by means of screws 16, with the interposition of sealing washers 18. Jacket 12 is seated on a shoulder 20 of flanges 15 and is sealed off by a sealing ring 22.

[0024] Further seen is an electromagnet 24, of which two are provided, one on each side of rolls 2 (see FIG. 6). Electromagnet 24 has a laminated, C-shaped core 26 and sloping pole faces 28. Windings 30 are made of copper tubing. Each of the poles of core 26 faces a laminated, annular armature 32, each armature being attached by means of screws 34 to flange 15 of the stub axle 8 of its roll 2.

[0025] Core 26 is slidably mounted on a bracket 35 (FIG. 1), which in turn is mounted on chassis 36. As the angle of slope of annular armatures 32 and the angle of slope of pole faces are identical, gap a between rolls 2, which determines the thickness of the cast sheet, can be altered without altering the air gap between armatures 32 and pole faces 28, by shifting core 26 in either direction of double arrow C. Windings 30 of electromagnet 24 are connected to a high-frequency (2.5-10 kHz) source of a voltage 38 (FIG. 6).

[0026] Returning to FIG. 1, there are seen coils 40, surrounding, but not touching, stub axles 8. Each of coils 40 has either one or two electrically separate windings. In the coil variant having only one winding, the DC and AC circuits are separated by a capacitor. When coils 40 are connected to sources of direct voltages 42 and low-frequency (5-100 Hz) alternating voltages 44 (FIG. 6), two magnetic fields are generated in annular magnetic circuits 32, one field being a direct and the other, a low-frequency, alternating magnetic field.

[0027] Magnetic core 46 is slipped over, but does not touch, two adjacent stub axles 8 and is fixedly attached to the coil former of coils 40. In order to accommodate alterations in the center distance d between the two adjacent rolls 2 due to alterations of gap a, core 46 is designed to be expanding. Expansibility is achieved by simply intercalating stacked laminations of core 46, as is clearly seen in FIG. 8. Holes 50 serve to allow stub axles 8 to pass through.

[0028] Stub axles 8 are mounted in bearings supported in bearing housings 52, which also serve as inlet (left housing) and outlet (right housing).

[0029] Rolls 2 are each connected, via coupling 54, to a drive 55 comprising a worm reduction gear 56 and an electric motor 58. Near the surface of rolls 2, arcuate stators 60 are installed, as shown in FIG. 2. These stators are connected to an m-phase (m≧3) voltage source 62 (FIG. 6) of controllable frequency, voltage and sequence of phase switching. Bearing housing 52 and drive 55 are mounted on beams 64, which are fitted with guides 66 consisting of female dovetails, guide rollers 68 freely rotating in bearing supports 70, and bracket 72 fixed to the bottom of frame beam 64 to which bracket nut 74 is attached. These are parts of the displacement mechanism of beam 64 with respect to chassis 36.

[0030] Chassis 36 includes guide rails 76 in the form of male dovetails matching the female dovetails of guides 66 on which the beams 64 rest, as well as screws 78 with handles 80 serving for the displacement of beams 64, and bracket 35 on which high-frequency electromagnets 24 are mounted. The working position of rolls 2 with respect to chassis 36, which determines the thickness of a cast sheet, is fixed using bolted joint 82.

[0031] Liquid metal is fed to the apparatus by a vacuum- or MHD-pump 84, through a thermally insulated metal conduit 86 and ceramic distribution box 88. The cast band or thin sheet is wound down using a puller strip 90, representing a steel band of a thickness corresponding to the thickness of the sheet to be case. Strip 90 has recesses at its upper end in which the crystallizing metal is caught; the strip serves as a leader for winder 92.

[0032] The continuous casting apparatus operates as follows: Using screws 78, the required gap between rolls 2 is set, then the position of beams 64 is rigidly fixed using bolted joints 82. Puller strip 90 is attached at its lower end to the drum of winder 92, its upper end being inserted into the gap between rolls 2 and held in position by guide rollers 68. Then, using voltages 38, 42, 44, currents are applied to the electromagnet windings, which excite direct magnetic field B₃ and alternating magnetic fields B₁ and B₂ in the zone of the anticipated location of the liquid layer end faces.

[0033] Then liquid metal is fed into the gap between rolls 2 by means of pump 84 via metal conduit 86 and distribution box 88 and, while the gap is being filled, rolls 2 are driven up by drive 55 to the nominal speed. The drum of winder 92 is driven up to a speed appropriate to the speed of rolls 2. At the appearance of liquid metal in the gap, vertical currents J₁, J₂ are induced at the edges of the liquid layer by alternating magnetic fields B₁, B₂. The interaction of these currents with these fields results in the appearance of electromagnetic body forces f₁, f₂, acting within a thin surface layer in the direction parallel to the axes of the rolls. These forces generate electromagnetic pressure, counterbalancing the hydrostatic pressure of the liquid column and thus preventing liquid metal overflow from the end faces of the liquid layer in the gap. However, these forces have an alternating component, which generates, together with the disturbing action of jets of liquid metal in the gap, wave disturbances on the free surface of the liquid layer. Free surface deviation in the direction perpendicular to the direct magnetic field B₃, induces a vertical current J₃, whose interaction with field B₃ leads to the appearance of electromagnetic body forces f₃, which are always directed counter to the motion of surface points and thus suppress wave excitations of the surface, making it more stable.

[0034] If the rate of liquid-metal feeding to rolls 2, the rate of rotation of the rolls and the intensity of roll cooling are well coordinated, the technological process of thin sheet (band) production is sufficiently stable.

[0035] The use of arcuate stators 60 together with jackets 12 (FIGS. 3b, 3 c) can make the technological process even more controllable, in the following way: A travelling magnetic field B₄, excited by arcuate stators 60, induces a travelling field of currents J₄ in rods 14, 14 a. Since rods lying in the diametral plane of rolls 2 are connected pair-wise, forming frames, travelling magnetic fields B₅ are induced in the liquid metal filled working gap, with the direction of motion being determined by the sequence of phases of arcuate stators 60. If the field B₅ moves in the direction opposite to that of the motion of the surface of rolls 2, the area of the contact surface of the liquid metal with the rolls increases and the thickness of the liquid layer in the central portion decreases, which increases the reliability of the liquid metal containment in the gap and permits an increase in the speed of rolls 2, i.e., the apparatus output. The use of arcuate stators 60, together with jackets 12 of FIG. 3b or 3 c as an additional electrical drive-enhancing electrical force, makes it possible to drastically reduce the power required by electrical drive 55.

[0036] It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 

1. An apparatus for the continuous casting of metal sheets and bands, comprising: two rotatable roll-crystallizers having parallel axes located in the horizontal plane and defining between them a gap of adjustable width, and means for supplying metal melt to fill said gap; characterized by means for generating at least three substantially horizontal magnetic fields producing electromagnetic pressures adapted to act to contain at least one end face of said melt within said gap.
 2. The apparatus as claimed in claim 1, wherein each of said rolls-crystallizers comprises a central, substantially cylindrical portion having a core made of non-ferromagnetic steel and two stub axles, one on each side of said cylindrical portion and made of a ferromagnetic steel.
 3. The apparatus as claimed in claim 2, further comprising a tubular jacket for each of said cylindrical portions, said jacket being made of a material having high thermal conductivity and being provided at its inside with grooves communicating with ducts inside said core and said stub axles, for the circulation of a liquid coolant.
 4. The apparatus as claimed in claim 3, wherein said jackets are provided on their outside surfaces with grooves extending in an axial direction, into which grooves are inserted electrically insulated copper bars, with pairs of bars located in diametrical planes being connected to one another inside said core, forming frames.
 5. The apparatus as claimed in claim 1, further comprising two electromagnets, each facing one end of said gap and consisting of a C-shaped, laminated core having two sloping pole faces, said electromagnets being adapted to generate a high-frequency, alternating magnetic field.
 6. The apparatus as claimed in claim 1, wherein each of said sloping pole faces of said C-shaped core is disposed adjacent to the sloping face of one of two laminated, annular armatures fixedly attached to a flange of said stub axles.
 7. The apparatus as claimed in claim 1, wherein each of said stub axles of each of said roll-crystallizers is surrounded by a coil connectable to a source of alternating current and a source of direct current, and adapted to generate a low-frequency, alternating magnetic field and a direct magnetic field.
 8. The apparatus as claimed in claim 1, wherein said roll-crystallizers are each mounted in bearings attached to a beam of their own, which beams are slidably mounted on a chassis, whereby said gap can be adjusted by using screw means to vary the distance between said beams.
 9. The apparatus as claimed in claim 1, further comprising pump means and a conduit for feeding said melt into said gap via a distribution box.
 10. The apparatus as claimed in claim 1, further comprising a gear motor for each of said roll-crystallizers, each motor being connected to its roll-crystallizer via a coupling.
 11. The apparatus as claimed in claim 1, further comprising two arcuate stators surrounding said central portions of said roll-crystallizers with an air gap of at most 1 mm and covering part of the circumference of said portions, said stators having a width of between 0.5 to 0.9 of the length of said portions.
 12. The apparatus as claimed in claim 11, wherein said stators are supplied by a low-frequency (5-100 Hz) alternating m-phase (m≧3) voltage source and are adapted to produce a travelling magnetic field.
 13. The apparatus as claimed in claim 1, further comprising a take-up winder for the cast sheets emerging from said gap.
 14. A method for the continuous casting of metal sheets and bands, said method comprising the steps of: providing means for generating at least three horizontal magnetic fields, a first, low-frequency, alternating field of a frequency between 5 and 100 Hz, a second, high-frequency, alternating magnetic field of a frequency between 2.5 and 10 kHz, and a third, direct magnetic field; providing coolable, counter-rotating rolls defining between them an adjustable gap for determining the thickness of the cast sheets and means for delivering metal melt into said gap; coordinating the rate of delivery of said melt, the rate of rotation of said rolls, the rate of cooling of said rolls and the intensity of said magnetic fields; switching on said magnetic fields, and pouring said melt into said gap.
 15. The method as claimed in claim 14, including the further step of: providing means for generating a travelling magnetic field in the liquid metal filling the gap between said rolls, and varying the parameters of said travelling magnetic field to vary the area of the contact surface of the liquid metal with said rolls. 